1. Field of the Invention
The present invention relates to a 2-dimensional electron gas field effect transistor (2DEGFET).
2. Brief Description of the Related Art
2DEGFET devices, for example, were reported in the IEEE Electron Device Letters, Vol. EDL-7, No. 12 (1986), p. 649, by Henderson.
A prior art 2DEGFET device will be explained with reference to FIG. 1 showing a partial cross-sectional view, FIG. 2(a) showing a distribution diagram of the Al composition ratio and the In composition ratio in the direction from an N-type AlGaAs layer to an undoped GaAs buffer layer, and FIG. 2(b) showing a potential band diagram.
In the 2DEGFET device, an undoped GaAs layer 2 acting as a buffer layer, an undoped In.sub.0.15 Ga.sub.0.85 As channel layer 3c, and an N-type Al.sub.0.15 Ga.sub.0.85 As layer 4 acting as a carrier supplying layer are formed on a semi-insulating GaAs substrate.
Two-dimensional electron gas (2DEG) is induced near by the interface between the AlGaAs layer 4 and an InGaAs layer 3c to form a channel. A cap layer 5 of N-type GaAs is formed on the AlGaAs layer 4. The source electrode 6a and drain electrode 6b are formed on the cap layer 5 through a vapor deposition process to obtain ohmic contacts to the 2DEG channel layer. The gate electrode 7 is formed in a recess portion which is formed by etching the cap layer 5.
The effect to be expected from InGaAs used for the channel layer is that an increased conduction band offset between the AlGaAs electron supplying layer and the channel layer results in an increased sheet electron concentration while a decreased electron effective mass results in increased electron mobility.
InGaAs and GaAs have a different lattice constant from each other. It has been well-known that a good interface can be formed by making the In.sub.x Ga.sub.1-x As channel layer to less than the critical film thickness which causes a misfit dislocation whereby the elastic strain is converted into a strain lattice layer which can relax lattice unconformity.
Since an increasing In composition ratio increases the lattice unconformity, the critical film thickness decreases.
In case the film thickness is thinned to suppress misfit dislocation generated with an increased In composition ratio, the sub-band energy within the quantum well increases inversely proportional to the second power of film thickness. As a result, a fall of electron concentration as well as a fall of carrier enclosure within an InGaAs well occur, thus eliminating the merit obtained by using InGaAs as a channel.
That is, the more the In composition in the InGaAs strain layer is increased to improve the sheet electron concentration and electron mobility, the more the lattice dislocation increases, falling into a dilemma in that restriction on the thickness of the channel layer becomes rigid because of the critical film thickness.
For that reason, usually an InGaAs channel layer with an In composition ratio of 0.15 and with a thickness of about 150 .ANG. being less than the critical film thickness has been used in the 2DEGFET, resulting however in a problem that increase of sheet electron concentration and improved electron transfer characteristics by increasing In composition ratio cannot be sufficiently exhibited.